Greenhouse experiments were conducted to examine the effect of glyphosate on reproductive development in sicklepod. Glyphosate was applied postemergence over the top at 112 and 280 g ai/ha to sicklepod at 4-leaf stage (L), 8-L, 4-L followed by 8-L, and 12-L. A nontreated control was included. Immediately after the 12-L application, number of flowers was recorded for all treatments twice per week for 8 wk. Pollen viability was measured on 1 open flower/plant/sampling time using Alexander stain. The number of pods, pod length, seeds per plant, 50-seed weight, total seed weight, seed germination, seed viability, and dry weight of aboveground biomass were also recorded. No significant differences among the treatments were found for average pod length, 50-seed weight, seed germination, seed viability, and aboveground biomass. The nontreated had 18 flowers counted over 8 wk. Glyphosate applied at 12-L and sequentially at 4-L and 8-L, averaged over glyphosate rates, reduced cumulative flower production after 8 wk by 65 and 54%, respectively, compared with the nontreated. Similarly, glyphosate at 280 g/ha, averaged over treatment timings, reduced flower production by 58% compared with the nontreated. Because the number of flowers produced was limited by glyphosate treatment due to flower abscission, pollen viability measurements could not be analyzed because of large numbers of missing data points. The number of pods, seeds, and total seed weight were reduced by 79, 80, and 81%, respectively, with 280 g/ha of glyphosate compared with the nontreated.

The use of photosystem II (PSII)-inhibitor and/or protoporphyrinogen oxidase (PPO)-inhibitor PRE herbicides in soybean may, under adverse environmental conditions, result in early season crop injury. A field study was conducted near Brule and North Platte, Nebraska, during the 2016 and 2017 growing seasons with the objective to evaluate the impact of PRE herbicides metribuzin (PSII-inhibitor) and sulfentrazone (PPO-inhibitor) on early season soybean development, final plant stand, and yield using 22 soybean varieties adapted to southwestern Nebraska. Herbicide treatments consisted of metribuzin (560 g ai ha−1) and sulfentrazone (280 g ai ha−1) applied within 3 d after planting and a nontreated control (NTC). Sulfentrazone reduced green canopy vegetation at the V2 growth stage by 22% and final plant stand at physiological maturity by 10% compared with the NTC. The number of pods per plant was 16% higher for sulfentrazone and the number of seeds per plant was 15% and 4% higher for sulfentrazone and metribuzin compared with the NTC, respectively. Sulfentrazone and metribuzin resulted in a slightly higher yield (3%) compared with the NTC, thus no yield reduction from PRE herbicides was observed in this study. These results support other findings that sulfentrazone and metribuzin have potential to cause early-season crop injury; however, when applied according to their label recommendations and following regional agronomic management practices, this impact may not translate into soybean yield reduction while such herbicides provide effective soil residual weed control.

2,4-dichlorophenoxyacetic acid (2,4-D) is a common ingredient in POST broadleaf herbicides labeled for use in turf, pastures, rangeland, and grain crops. The herbicide 2,4-D is a weak acid, and when dissociated can bind to cations present in hard-water spray solutions and/or fertilizer solutions. Experiments were conducted with 2,4-D dimethylamine to evaluate the effect of cation solutions on herbicide efficacy on the perennial broadleaf weeds dandelion and broadleaf plantain. The objectives of this research were to (1) determine if 2,4-D efficacy is influenced by the divalent cations, calcium (Ca), magnesium (Mg), manganese (Mn), and zinc (Zn) in spray solution; and (2) determine if adding the adjuvant ammonium sulfate (AMS) to the spray solution can overcome antagonism. Broadleaf plantain and dandelion control was reduced and plant size and mass increased when 2,4-D was applied in a Ca solution in comparison to deionized water. However, 2,4-D antagonism was overcome when AMS was added as an adjuvant to the spray solution. Magnesium caused 2,4-D antagonism on both weed species in one run of the experiment similar to Ca solution and AMS was successful at overcoming antagonism when added to the tank mixture. Some 2,4-D antagonism from Mn was noticed even when AMS was in the tank mix, but Zn fertilizer solutions did not antagonize 2,4-D activity on either weed species. Although divalent cations can antagonize 2,4-D dimethylamine and reduce perennial broadleaf weed control, adding AMS can overcome this antagonism when Ca and Mg are the primary cations in spray solution. Applicators should avoid using Mn fertilizers when applying 2,4-D dimethylamine because AMS did not successfully overcome antagonism.

Field experiments were conducted in 2008, 2011, and 2012 to investigate the interaction between seeding system disturbance and PPI herbicides on rigid ryegrass control in wheat. Of the herbicides examined, prosulfocarb + S-metolachlor, and pyroxasulfone provided ≥ 70% control of rigid ryegrass, irrespective of seeding system. In contrast, trifluralin was the least-effective herbicide against rigid ryegrass and was particularly ineffective when used with single disc (10% control) relative to the triple-disc seeding system (80%) in 2012. Trifluralin consistently reduced wheat density when incorporated using single discs (46 to 59%) but not do so with the triple disc or double-shoot knife-point systems. Although there were large differences in crop establishment because of herbicide phytotoxicity, that did not always translate into large differences in yield because wheat was able to recover from reductions in plant density by increasing the spike number per plant. Pyroxasulfone caused no damage to wheat and appeared to be the most suitable PPI herbicide for use with single-disc seeding systems.

Failure to control Palmer amaranth with glyphosate and protoporphyrinogen IX oxidase (PPO)-inhibitor herbicides was reported across southwestern Nebraska in 2017. The objectives of this study were to 1) confirm and 2) validate glyphosate and PPO-inhibitor (fomesafen and lactofen) resistance in 51 Palmer amaranth accessions from southwestern Nebraska using genotypic and whole-plant phenotypic assay correlations and cluster analysis, and 3) determine which agronomic practices might be influencing glyphosate resistance in Palmer amaranth accessions in that location. Based on genotypic assay, 88% of 51 accessions contained at least one individual with amplification (>2 copies) of the 5-enolypyruvyl-shikimate-3-phosphate synthase (EPSPS) gene, which confers glyphosate resistance; and/or a mutation in the PPX2 gene, either ΔG210 or R128G, which endows PPO-inhibitor resistance in Palmer amaranth. Cluster analysis and high correlation (0.83) between genotypic and phenotypic assays demonstrated that EPSPS gene amplification is the main glyphosate resistance mechanism in Palmer amaranth accessions from southwestern Nebraska. In contrast, there was poor association between genotypic and phenotypic responses for PPO-inhibitor resistance, which was attributed to segregation for PPO-inhibitor resistance within these accessions and/or the methodology that was adopted herein. Genotypic assays can expedite the process of confirming known glyphosate and PPO-inhibitor resistance mechanisms in Palmer amaranth from southwestern Nebraska and other locations. Phenotypic assays are also a robust method for confirming glyphosate resistance but not necessarily PPO-inhibitor resistance in Palmer amaranth. Moreover, random forest analysis of glyphosate resistance in Palmer amaranth indicated that EPSPS gene amplification, county, and current and previous crops are the main factors influencing glyphosate resistance within that geographic area. Most glyphosate-susceptible Palmer amaranth accessions were found in a few counties in areas with high crop diversity. Results presented here confirm the spread of glyphosate resistance and PPO-inhibitor resistance in Palmer amaranth accessions from southwestern Nebraska and demonstrate that less diverse cropping systems are an important driver of herbicide resistance evolution in Palmer amaranth.

Field experiments were conducted from 1987 to 1990 to investigate the combination of cultural and chemical methods for control of Amaranthus spp. in corn. The tolerance of corn hybrids to herbicides or herbicide combinations was dependent on application rate. The early season threshold level for a mixed population of Amaranthus spp. occurred 3.5 wk after emergence. Density of Amaranthus spp. from 155 to 495 plants/m2 caused corn yield reduction of 50%. A quadratic model was derived relating percentage of yield reduction vs. duration of weed competition. The results emphasize the importance of using herbicides, cultural practices, and competition thresholds for an integrated weed control approach of Amaranthus spp. in corn.

Chlorimuron, clomazone, imazaquin, imazethapyr, and pendimethalin were each applied at five rates to soils at 10 different sites each year for three years to determine which soil properties influenced their bioactivity. Six soils in the U.S. (NC) and four in Zimbabwe were characterized for their plow-layer contents of organic matter (OM), humic matter (HM), clay mineral (CM), and silt and for pH and cation exchange capacity (CEC). At each site, the rate of each herbicide yielding 80% weed control (I80) was determined by visually evaluating the treated plots. The I80 values were then regressed with the various soil properties to determine which of them contributed to this rate. In the U.S. soils, the I80 was highly correlated with % OM (r 2 = 0.64–0.72) and % HM (r 2 = 0.55–0.69) for chlorimuron, imazaquin, and pendimethalin but was less correlated with % OM (r 2 = 0.30) and pH (r 2 = 0.31) for imazethapyr. Equations relating the herbicide rate for 100% weed control to soil properties were derived for chlorimuron, imazaquin, imazethapyr, and pendimethalin then compared with registered recommended rates for each chemical from the label. Herbicide rate equations based on similar soil parameters were taken from the literature to compare with our rate equations and with registered rate recommendations for comparison purposes and to add validity to the use of soil parameters for making herbicide rate recommendations. The rate equations, based on selected soil parameters, produced rates comparable with registered rates for chlorimuron, pendimethalin, and metribuzin and lower rates of application than registered rates for imazaquin, imazethapyr, alachlor, and metolachlor for soils with OM levels below 3.5%.

Weed management experiments with kidney beans were conducted at one irrigated site in 1991 and at two irrigated sites near Staples, MN during 1992. Green foxtail was the most difficult weed to control at the three research sites which agrees with survey results from dry bean growers regarding their worst weed problems. Hairy nightshade increased rapidly when weed management methods were reduced. Dry bean seed yields were highest in cultivated check plots on the farmer's field and in conventional practices plots at the Irrigation Research Center. Net economic returns, averaged over 1991 and 1992, for producing kidney beans at the Irrigation Research Center were $899/ha using conventional practices, $791/ha with cover crop practices, $734/ha with cultivated checks, $690/ha with herbicides only, and $208/ha on weedy checks. Kidney beans are an economical crop for North Central growers, but adequate weed management technologies are necessary for profitable production.

Phenoxy herbicides are frequently used to control volunteer canola populations. However, there have been claims that poor control could be due to cold acclimation of canola plants in the spring. The objective of this study was to determine whether cold acclimation or growth stage affected the response of canola volunteers to herbicides. In a growth room experiment, canola plants were prehardened and cold acclimated or were grown at 20/12 C and treated with one of six 2,4-D doses. Cold acclimation as achieved by this experiment affected upper and lower asymptotes of the dose–response curve but not the herbicide dose required to reduce canola weight by 50% relative to the nontreated control (GR50), indicating limited cold-related effects on canola tolerance to 2,4-D. Field experiments, conducted in the provinces of Québec and Saskatchewan, examined the effects of canola growth stage on the efficacy of 2,4-D, MCPA, and carfentrazone. Comparisons of the estimates from the dose–response curves confirmed that herbicide efficacy was consistently greater when canola plants were treated at an early growth stage, regardless of cultivar or herbicide used. The GR50 estimates for canola plants treated at a later growth stage exceeded the recommended rates. Some canola plants grown as volunteers in a wheat crop survived 2,4-D or MCPA treatments at 0.5× and 1× rates and produced up to 148 seeds/m2. Efficient control of canola volunteers will be obtained when plants are sprayed at an early growth stage, but near-total control will be highly desirable in order to restrict seedbank buildup, particularly when dealing with canola cultivars with different herbicide-resistant traits.

The development of crop cultivars with resistance to selected herbicides has the potential to impact environmental quality, food safety, consumers, and crop producers in either a positive or negative manner. The technology that makes it possible to develop herbicide-resistant crops is neither good nor bad, it is rather how the products of this technology are used that will determine whether or not the introduction of herbicide-resistant crops is ultimately a good or bad decision. The introduction of herbicide-resistant crops will have diverse impacts leading to redundancy, diversity, and confusion in crop production systems. Often the introduction of herbicide-resistant cultivars will have the same impact on cropping systems as the introduction of a new herbicide that has the same mode-of-action and use pattern of herbicides already in use. This may add diversity of herbicide options for a given crop but will cause redundancy of product use over several years. This redundancy could lead to weed resistance and water quality concerns. Confusion at the user level will exist because not all cultivars of a crop will be resistant to the herbicide; this could be the major deterrent to widespread adoption of herbicide-resistant crops. Steps must be taken to provide information to crop producers that will insure that herbicide-resistant crops are used effectively and safely. Weed scientists will determine whether this technology will be used to improve food safety, water quality, crop production systems, and farmer profitability or have a negative impact on agriculture and the whole of society.

Efficacy of herbicide programs containing clomazone PPI plus fluometuron PRE or clomazone plus pendimethalin PPI plus fluometuron PRE was compared with that of standard programs of pendimethalin PPI plus fluometuron PRE and norflurazon PPI plus norflurazon and fluometuron PRE. Cotton injury was less than 5% with all treatments when disulfoton or phorate was applied in the seed furrow. Control of fall panicum, goosegrass, large crabgrass, eclipta, entireleaf morningglory, ivyleaf morningglory, pitted morningglory, tall morningglory, prickly sida, redroot pigweed, smooth pigweed, hemp sesbania, spotted spurge, sicklepod, and velvetleaf and cotton yields with 0.8 kg ai ha−1 of clomazone plus fluometuron or 0.6 kg ha−1 of clomazone plus pendimethalin plus fluometuron equalled or exceeded that from the standard herbicide programs. POST-directed application of methazole at 0.8 kg ai ha−1 plus MSMA at 2.2 kg ae ha−1 increased sicklepod and morningglory control and cotton yield. Clomazone applied PRE at 0.6 kg ha−1 with fluometuron controlled broadleaf signalgrass, goosegrass, large crabgrass, prickly sida, and smooth pigweed equally with that of standard treatments of trifluralin or trifluralin plus norflurazon PPI and fluometuron PRE, whereas pitted morningglory control and cotton yield with clomazone plus fluometuron exceeded that with the standards.

Field experiments were conducted in North Carolina from 1994 through 1998 to evaluate the effects of five weed management strategies and four corn (Zea mays)–peanut (Arachis hypogaea) rotation sequences on purple nutsedge (Cyperus rotundus) population development. Effects of these weed management programs on cotton (Gossypium hirsutum) and peanut production in following years were also investigated. Herbicide programs included a nontreated control, a carbamothioate preplant incorporated (PPI) combination treatment utilizing vernolate in peanut and butylate in corn, an early postemergence (EPOST) acetolactate synthase (ALS) inhibitor combination treatment utilizing imazapic in peanut and halosulfuron in corn, and EPOST treatments of imazapic and imazethapyr in both peanut and imidazolinone-resistant corn. Crop rotation sequences for the 3 yr included continuous corn (CCC), corn–peanut–corn (CPC), peanut–corn–peanut (PCP), and continuous peanut (PPP). The imazapic and ALS inhibitor combination treatments both provided excellent shoot and tuber control. After 3 yr, imazapic and the ALS inhibitor combination treatment reduced shoot and tuber population densities to less than 10% of the nontreated control. Imazethapyr provided variable but better control than the carbamothioate treatment with tuber densities (measured from 0 to 15 cm soil depth) and shoot densities increasing from 733 to 2,901 tubers/m3 of soil and 16 to 43 shoots/m2, respectively, after 3 yr. Tuber densities increased in the nontreated control from 626 to 9,145 tubers/m3 of soil and from 962 to 5,466 tubers/m3 of soil in the carbamothioate treatment during this same period. Also, shoot densities increased in the nontreated control from 22 to 159 shoots/m2 and from 8 to 92 shoots/m2 in the carbamothioate treatment. There was a 31% peanut yield reduction from 1994 to 1996 when peanut was continuously planted or rotated to corn for only 1 yr. Herbicide carryover effects were not observed in cotton during 1997.

Plant growth regulators were evaluated on common and African ‘Tifway’ bermudagrass. Flurprimidol plus mefluidide applied at 1.1 plus 0.14 kg ha-1 and followed by 0.56 plus 0.14 kg ha-1 at 2- to 3-week interval suppressed vegetative growth of mowed common bermudagrass for 5 wk (17 to 23%) and unmowed turf for 6 wk (40%), but severely injured the turf. The injury ranged from 25 to 32% from 2 to 6 wk after treatment. The plant growth regulators that injured common bermudagrass less than flurprimidol plus mefluidide did not suppress the mowed turf for as long a period. Vegetative growth of common bermudagrass not mowed was suppressed for 6 wk when treated once with flurprimidol plus mefluidide and twice with flurprimidol, mefluidide, imazethapyr, paclobutrazol (1.1 plus 1.1 kg ha-1), and paclobutrazol plus mefluidide. Of the plant growth regulators evaluated, only imazethapyr suppressed common bermudagrass seedheads. The suppression was 70% for 4 wk, but reduced to <70% by 5 wk. Paclobutrazol applied initially at 1.1 kg ha-1 and followed at 0.56 kg ha-1 suppressed vegetative growth of mowed Tifway bermudagrass for 5 wk and unmowed turf for 8 wk without causing severe injury. The suppression of mowed Tifway bermudagrass with two applications of paclobutrazol was as good or better than with any other plant growth regulator. All plant growth regulators suppressed vegetative growth of unmowed Tifway bermudagrass for 8 wk.

This study documents the physiology and genetics of evolved atrazine resistance in a wild radish population from Western Australia. Plant response to atrazine treatment confirmed a high level of resistance in population WARR5. At 0.25 kg atrazine/ha, all plants from a susceptible population were killed, whereas resistant WARR5 was unaffected at the highest dose tested (4 kg atrazine/ha). Leaf photosynthesis in susceptible plants was inhibited after 1 kg atrazine/ha treatment, whereas leaf photosynthesis in WARR5 plants was unaffected. Furthermore, atrazine resistance was maternally inherited. Sequencing of a psbA gene fragment in resistant WARR5 and susceptible plants revealed a single point mutation resulting in a coding change from Ser264 to Gly of the D1 protein in resistant plants. We are confident that this mutation is the basis of resistance to the photosystem II inhibitors in this wild radish population.

Asulam applied POST alone and in mixtures with residual herbicides and with DSMA and MSMA was evaluated for johnsongrass control. Johnsongrass control following treatment with asulam at 3.7 kg ai ha−1 was not improved by tank mixtures with the residual herbicides atrazine, metribuzin, pendimethalin, and terbacil compared with asulam applied alone. Johnsongrass injury symptoms appeared sooner when asulam was applied in mixtures with either DSMA or MSMA, but johnsongrass control 4 wk after treatment was similar to asulam applied alone. As a result of the johnsongrass control obtained with early POST treatments of asulam applied alone, cane and sugar yields were increased by 56 and 58%, respectively, when compared with the weedy check. Additional cane and sugar yield increases were not obtained by any of the asulam mixtures because johnsongrass control was not improved. Asulam applied again in May to johnsongrass regrowth improved late-season control in two of the three studies over a single early POST application, but cane and sugar yields were not increased.

Cogongrass is a highly invasive perennial grass that threatens agriculture, forestry, and native plant assemblages in many regions of the world. Cogongrass is a prolific seed producer, but the ecological importance of seeds as a vector of invasion has not been adequately addressed. Propagule pressure plays a key role in many successful invasions and the relative importance of propagule pressure across environmental gradients is an area of research that is quickly becoming important in understanding invasion success. The present study was conducted to test the effects of varying propagule density upon the ability of cogongrass seedlings to grow and establish across experimentally manipulated light and soil gradients. Seedlings growing in high-nutrient soil performed the best regardless of the amount of available light, but overall biomass was always greatest in high-light environments. Cogongrass showed a very strong trend in biomass allocation, with seedlings in reduced light environments partitioning more biomass to aboveground tissue. The data provided no evidence that initial propagule density affected the growth and establishment of cogongrass seedlings, but growth and biomass production were positively correlated with final seedling density.

In 2004 and 2005, field research was conducted in Idaho to compare tank mixtures of flumioxazin at the 53 g ai/ha potato use rate with comparable tank mixtures of rimsulfuron at 26 g ai/ha for broad-spectrum weed control. Flumioxazin in two-way tank mixtures with metribuzin, EPTC, pendimethalin, S-metolachlor, or ethalfluralin provided greater than 90% hairy nightshade control, which was comparable with control by similar rimsulfuron two-way tank mixtures. Flumioxazin plus metribuzin or rimsulfuron were the only two-way mixtures with flumioxazin consistently providing 90% or greater redroot pigweed, common lambsquarters, and green foxtail control. Control of these weeds by any of the rimsulfuron two-way mixtures was almost always greater than 90%. Three-way tank mixtures containing flumioxazin or rimsulfuron controlled hairy nightshade, redroot pigweed, and common lambsquarters similarly, and control ranged from 89 to 100%. When metribuzin was not included with flumioxazin in three-way mixtures, control was 80 to 97% and not always comparable with the 89 to 100% control by similar rimsulfuron mixtures. Green foxtail control by flumioxazin or rimsulfuron three-way mixtures usually was similar and greater than 90%.

A fixed-plot management study for control of acetolactate synthase (ALS)–resistant common cocklebur in soybean was initiated in 1994 at Fayetteville, AR. Three susceptible and three imazaquin-resistant common cocklebur plants were transplanted into the field, and seed (burs) were distributed throughout the plots in the fall of 1994. Herbicide treatments included imazaquin, chlorimuron, and chlorimuron plus metribuzin applied each year from 1995 through 1999 and herbicide rotations containing ALS inhibitors and herbicides with alternative modes of action. Effectiveness of management systems and the dynamics of the development of common cocklebur resistance, including development of resistance to imazaquin and chlorimuron, were evaluated. Imazaquin controlled susceptible common cocklebur populations in 1995 but not the resistant population, resulting in significant soybean yield reduction. By the end of the 1996 season, the resistant biotype dominated imazaquin plots, and a high level of cross-resistance to chlorimuron was observed in the population. Resistant populations were reduced by non-ALS herbicide programs of sulfentrazone plus clomazone applied preemergence (PRE), metribuzin plus clomazone applied PRE followed by bentazon applied postemergence (POST), and transgenic herbicide programs of glyphosate and glufosinate applied POST. Rotating ALS inhibitors with non–ALS-inhibiting heribicides may slow the development of resistance, but resistant individuals may eventually dominate the population.

Understory vegetation was studied following treatment by herbicides and fire on grazed pastures within the Cross Timbers vegetation type. Tebuthiuron and triclopyr at 2.2 kg ha-1 were applied in March and June of 1983, respectively. The pastures were burned in late spring of 1985, 1986 and 1987. Frequency of horseweed, rosette panicgrass, and little bluestem increased with both herbicides, but the increase was usually larger with tebuthiuron than triclopyr. Burning as a follow-up to either herbicide had little effect on frequency of rosette panicgrass and little bluestem. Horseweed increased following burning in 1985 and 1987. Standing crop of grasses and forbs increased dramatically following herbicide treatments. Grass production was greater with tebuthiuron, whereas production of forbs and browse was generally greater with triclopyr.

A 3-yr study was conducted on nine farms across southern Ontario to evaluate the risks and benefits of different approaches to weed management in corn and soybean. Weed control decisions were based on field scouting and recommendations from the Ontario version of HADSS™, the herbicide application decision support system. Treatments were selected to maximize profit (economic threshold approach) or to maximize yield (highest treatment efficacy). Reduced rates of the high efficacy treatment for each field also were included. Weed density before and after treatment, crop yields, weed seed return, and the effect of weed control decisions on weed density 1 yr after treatment were assessed. Crop yield varied among years and farms but was not affected by weed control treatment. Weed control at 28 d after treatment (DAT) was often lower and weed density, biomass, and seed production 70 DAT were often higher with the profit maximization approach compared with the yield maximization approach. However, weed density 1 yr later, after each cooperator had applied a general weed control program, did not vary significantly among the previous year's weed control treatments. Reduced rates of the high efficacy treatments did not lead to increased weed problems the next year, despite lower weed control and increased weed seed production in some years. During the 3 yr of the study, weed control costs with the profit maximization approach were approximately Can$45/ha less than with the yield maximization approach.